Methods are known for operating an internal combustion engine having a compressor, especially a turbocharger. A pressure ratio across the compressor is limited by means of a pump limit in dependence upon a mass flow through the compressor in order to prevent a pumping of the compressor. If the pressure ratio exceeds the pump limit, then this leads to an intense oscillation of the fresh air mass supplied to the engine. This becomes noticeable from a pulsating noise which is based on the so-called charger pumping or compressor pumping. The pump limit defines the limit of a compressor characteristic field starting from which the compressor characteristic field is no longer defined. The compressor characteristic field determines the permissible pressure ratio across the compressor in dependence upon the mass flow through the compressor. In FIG. 3, a compressor characteristic field is shown by way of example wherein the pressure ratio is plotted against a corrected compressor mass flow in pounds/min. In the compressor characteristic field of FIG. 3, isolines of constant engine rpm nmot of the engine are presented by way of example as well as isolines of constant rpm nv of the compressor of a turbocharger. With respect to the engine rpm nmot, isolines for 1,000, 1,500 and 2,000 rpm and, for the compressor rpm nv, isolines having 85,000, 105,000, 125,000, 145,000 and 165,000 rpm are plotted. The pump limit is shown by a broken line in the compressor characteristic field of FIG. 3. The pump limit line intersects with the isoline for the engine rpm nmot=1,000 rpm. When, for example, the compressor rpm nv=145,000 rpm and the engine rpm nmot=1,500 rpm, then the pressure ratio lies below the pump limit and therefore in a permissible range. For this reason, there is no compressor pumping. When the internal combustion engine drives a vehicle and the driver shifts into the next higher gear whereat the engine rpm nmot drops to 1,000 rpm, this means, for a constant compressor rpm nv=145,000 rpm, a change of the pressure ratio beyond the pump limit to the isoline for nmot=1,000 rpm. The pressure ratio across the compressor would thereby no longer be in the permissible range so that compressor pumping would occur. For this reason, in this case, the pressure ratio on the isoline nmot=1,000 rpm must be dropped until it is again in the permissible range, that is, it lies below the pump limit. For this purpose, the compressor rpm nv can be correspondingly reduced to a value which is approximately 115,000 rpm or less.
The pressure ratio is defined as the ratio of the pressure in flow direction downstream of the compressor to the pressure in flow direction forward of the compressor. The compressor mass flow plotted along the abscissa is corrected in dependence upon the pressure and the temperature of the compressor mass flow in flow direction forward of the compressor. The pressure in flow direction downstream of the compressor is characterized as charge-air pressure and the pressure in flow direction forward of the compressor is characterized as intake pressure. There are two possibilities for triggering the compressor pumping. For constant charge-air pressure but low ambient pressure, for example at high elevation, clearly higher compressor pressure ratios result when one proceeds from the consideration that the intake pressure corresponds approximately to the ambient pressure. In this way, the pressure ratio across the compressor can increase beyond the pump limit so that a compressor pumping is triggered. This compressor pumping is characterized as static compressor pumping. The static compressor pumping can be prevented in that a desired value for the charge-air pressure is so selected over all operating ranges of the internal combustion engine that the pressure ratio even for low ambient pressures has a safety spacing from the pump limit. Unit scattering and deterioration effects must be considered for the dimensioning of the safety spacing. In this way, the operating range of the compressor in the compressor characteristic field is limited.
For a rapidly falling engine rpm nmot, a combination of low engine rpm nmot and a high compressor rpm nv can occur which likewise can effect a pressure ratio above the pump limit and therefore a compressor pumping and is characterized as a dynamic compressor pumping.